Tachyarrhythmias are abnormal heart rhythms characterized by a rapid heart rate, typically expressed in units of beats per minute (bpm). Examples of tachyarrhythmias include supraventricular tachycardias (SVT's) such as sinus tachycardia, atrial tachycardia (AT), and atrial fibrillation (AF). The most dangerous tachyarrythmias, however, are ventricular tachycardia (VT) and ventricular fibrillation (VF). Ventricular rhythms occur when re-entry of a depolarizing wavefront in areas of the ventricular myocardium with different conduction characteristics becomes self-sustaining or when an excitatory focus in the ventricle usurps control of the heart rate from the sinoatrial node. The result is rapid and ineffective contraction of the ventricles out of electromechanical synchrony with the atria. Most ventricular rhythms exhibit an abnormal QRS complex in an electrocardiogram because they do not use the normal ventricular conduction system, the depolarization spreading instead from the excitatory focus or point of re-entry directly into the myocardium. Ventricular tachycardia is typically characterized by distorted QRS complexes that occur at a rapid rate, while ventricular fibrillation is diagnosed when the ventricle depolarizes in a chaotic fashion with QRS complexes of constantly changing shape. Both ventricular tachycardia and ventricular fibrillation are hemodynamically compromising, and both can be life-threatening. Ventricular fibrillation, however, causes circulatory arrest within seconds and is the most common cause of sudden cardiac death.
Cardioversion (an electrical shock delivered to the heart synchronously with the QRS complex) and defibrillation (an electrical shock delivered without synchronization to the QRS complex to terminate ventricular fibrillation) can be used to terminate most tachyarrhythmias, including SVT's, VT, and VF. The electric shock terminates the tachyarrhythmia by depolarizing all of the myocardium simultaneously and rendering it refractory. A class of cardiac rhythm management devices known as an implantable cardioverter/defibrillator (ICD) provides this kind of therapy by delivering a shock pulse to the heart when the device detects fibrillation.
Another type of electrical therapy for tachycardia is anti-tachycardia pacing (ATP). In ATP, the heart is competitively paced with one or more pacing pulses in an effort to interrupt the reentrant circuit causing the tachycardia. ATP can be applied to either the ventricles or the atria. Modern ICD's typically have ATP capability so that ATP therapy is delivered to the heart when a tachycardia is detected, while a shock pulse is delivered when fibrillation occurs. Although cardioversion/defibrillation will terminate tachycardia, it consumes a large amount of stored power from the battery and results in patient discomfort owing to the high voltage of the shock pulses. It is desirable, therefore, for the ICD to use ATP to terminate a tachyarrhythmia whenever possible. It is commonly believed that only cardioversion/defibrillation will terminate fibrillation and certain high rate tachycardias, while ATP can be used to treat lower rate tachycardias. A tachyarrhythmia that is regarded as terminable by ATP therapy, based upon rate or other factors, will be referred to herein as either a terminable tachyarrhythmia or a tachycardia.
In most ICD's with ATP capability, fibrillation (VF or AF) is distinguished from tachycardia (VT or AT) using rate-based criteria so that ATP or shock therapy can be delivered as appropriate. The ventricular heart rate is usually measured by detection of the time between successive R waves (i.e., ventricular depolarizations), and the atrial rate is measured by detection of the time between successive P waves (atrial depolarizations). A measured heart rate is classified as a tachycardia when the rate is in a tachycardia zone, defined as a range of rates above a tachycardia detection rate (TDR) but below a fibrillation detection rate (FDR). A measured heart rate above the FDR, on the other hand, is in the fibrillation zone and is classified as either atrial or ventricular fibrillation. In a typical device, a tachyarrhythmia with a heart rate in the tachycardia zone is treated with ATP therapy in order to avoid an unnecessary painful shock to the patient, and a defibrillation shock is delivered if the pacing fails to terminate the tachyarrhythmia. The present disclosure relates to a method and apparatus for delivering ATP therapy in a manner that increases the likelihood that ATP therapy will terminate a tachyarrhythmia without resorting to a defibrillation shock.